1. Field of the Invention
The present invention relates to a surface-mount quartz crystal oscillator, and more particularly to a surface-mount crystal oscillator which is highly resistant to shock and highly productive.
2. Description of the Related Art
Surface-mount quartz crystal oscillators which have a quartz crystal unit and an oscillating circuit using the crystal unit, both housed in a surface-mount casing, are small in size and widely used as a frequency or time reference source, for example, in portable devices and the like. One application of such a surface-mount crystal oscillator is as a crystal oscillator serving as a synchronizing signal source, i.e., so-called SPXO (simple packaged crystal oscillator). Surface-mount crystal oscillators which are presently dominant in the art have planar profile dimensions of 7 mm×5 mm.
FIGS. 1A and 1B show a conventional surface-mount crystal oscillator by way of example. As shown in FIGS. 1A and 1B, the conventional surface-mount crystal oscillator comprises casing 1 having a substantially rectangular parallelepiped as an outer shape and planar profile dimensions of 7 mm×5 mm, for example, IC (integrated circuit) chip 2, quartz crystal blank 3, and cover 9 sealing a recess defined in casing 1. Casing 1 is made of laminated ceramic and has the recess defined therein. Casing 1 which is made of the laminated ceramic is of a four-layer structure made up of bottom wall layer 1a, first intermediate layer 1b, second intermediate layer 1c, and top wall layer 1d. Of these layers, first intermediate layer 1b, second intermediate layer 1c, and top wall layer 1d have respective substantially rectangular openings defined therein, which are progressively larger in the named order of the layers. As a result, the openings jointly serve as the recess in casing 1, providing two steps, i.e., upper steps 4a and lower steps 4b, on the inner wall of the recess. Upper steps 4a are positioned at opposite ends of the recess in a longitudinal direction of casing 1. Lower steps 4b are positioned at the opposite ends of the recess in the longitudinal direction of casing 1 and also at opposite ends of the recess in a transverse direction of casing 1, and hence extend in a frame manner in the recess. One of upper steps 4a has a central trench extending downwardly to the upper surface of corresponding lower step 4b, and is divided into two segments by the central trench. Electrically conductive pads are disposed on upper surfaces of lower steps 4b which are positioned at the opposite ends of the recess in the longitudinal direction of casing 1. Connecting electrodes, not shown, for electrical and mechanical connection to crystal blank 3 are disposed on the upper surfaces of the divided segments of upper step 4a. Although not shown, mounting terminals extend from an outer bottom surface to a side surface of casing 1, and serve to provide electrical connection to a circuit pattern on a wiring board. The mounting terminals and the electrically conductive pads disposed on lower steps 4b are electrically connected to each other by through holes or the like defined in casing 1, and the electrically conductive pads and connecting terminals on upper steps 4a are electrically connected to each other by a circuit pattern or the like disposed in the recess in casing 1.
IC chip 2 is electrically connected to crystal unit 3, and provides an oscillating circuit using crystal unit 3. IC chip 2 contains an integrated assembly of an oscillating amplifier and capacitors. Terminals 12 including a power supply terminal, ground terminal, and output terminal are mounted on opposite ends of one of principal surfaces of IC chip 2. The other principal surface of IC chip 2 is fixed to the bottom of the recess in casing 1, i.e., an upper surface of bottom wall layer 1a. Electrically conductive pads 11 on the upper surfaces of lower steps 4b and terminals 12 on IC chip 2 are connected by gold wires 5 according to a wire bonding process. Gold wires 5 extend substantially in the longitudinal direction of casing 1. IC chip 2 has a planar profile size which is typically of 1.2 mm×1.3 mm. IC chip 2 as mounted in casing 1 has its longitudinal axis aligned with the longitudinal direction of casing 1.
As shown in FIG. 1C, crystal blank 3 is of a substantially rectangular shape, for example, and comprises an AT-cut quartz crystal blank. Crystal blank 3 has a planar size of 5 mm×3 mm, for example. The AT-cut crystal blank has its resonant frequency determined by its thickness. Crystal blank 3 has a pair of excitation electrodes 6 disposed respectively on opposite principal surfaces thereof. From excitation electrodes 6, there extend respective extension electrodes 7 toward respective opposite ends of one side of crystal blank 3. In the shown example, extension electrodes 7 extend the opposite ends of one shorter side of crystal blank 3. The opposite ends of the one side of crystal blank 3 to which extension electrodes 7 extend are bonded to the upper surfaces of upper step 4a where the connecting electrodes are disposed, by electrically conductive adhesive 8. The other end of crystal blank 3, which opposes to the one side of crystal blank 3, is placed in abutment against the upper surface of upper step 4a which is not divided. Crystal blank 3 is thus held in casing 1 and electrically connected to casing 1. The end of crystal blank 3 which is bonded by electrically conductive adhesive 8 acts a fixed end, whereas the end of crystal blank 3 opposite to the fixed end as a free end.
Since IC chip 2 and electrically conductive pads 11 are connected to each other by gold wires 5 according to a winding bonding technique on longitudinally opposite ends of casing 1, and gold wires 5 extend in the longitudinal direction of casing 1, a space can easily be provided for wire bonding operation in this crystal oscillator. For example, a capillary of the wire bonding apparatus can easily be guided to lower steps 4b. As the other end, i.e., the free end, of crystal blank 3, is placed on upper step 4a, the one end, i.e., the fixed end, of crystal blank 3, can easily be bonded in position by electrically conductive adhesive 8, and the free end is prevented from vertically swinging for protection against damage to crystal blank 3 when external mechanical shocks are applied to the crystal oscillator. Therefore, the crystal oscillator can be manufactured highly efficiently and is highly resistant to shocks.
As the oscillating frequency of the surface-mount crystal oscillator goes higher, i.e., is in a range from 100 to 170 MHz, AT-cut crystal blank 3 has a much smaller thickness, and tends to be fractured. As a result, the free end of crystal blank 3 which is merely placed on upper step 4a is not sufficiently strong to prevent crystal blank 3 from being broken. It has been attempted to coat cover 9 with an insulative adhesive over the free end of crystal blank 3, forming a protrusion projecting into the recess in casing 1 to reduce the range of vertical swinging of crystal blank 3. However, forming such a protrusion on cover 9 makes the assembly process longer and more complex. In addition, because the protrusion tends to secure crystal blank 3 substantially at its opposite ends, stresses are liable to concentrate on a central region of crystal blank 3, crystal blank 3 is likely to be broken at its central region.